The present invention relates generally to methods and systems for testing radio frequency identification (RFID) devices and relates more particularly to a novel method and system for testing RFID devices.
The ability to retrieve information regarding an object of interest in an efficient and wireless manner is critical in many types of situations. For example, many current manufacturing and distribution methods require wireless techniques for retrieval of information regarding objects of inventory, which retrieval techniques can be used, for example, to track the location of an object of inventory from the time of its manufacture to the time of its sale to a customer. One well-known wireless technique for retrieving information relating to an object involves coupling to the object a radio frequency identification (RFID) device that stores pertinent information relating to the object and that wirelessly communicates such information to an electronic reader in response to a wireless interrogation. The types of information that may be stored on the RFID device may include, for example, a unique identification number, an expiration date, a “born on” date, manufacturing information, shipment status, pricing information and the like.
One well-known type of RFID device comprises an antenna and an integrated circuit (IC) chip mounted on the antenna, the IC chip being programmed to store the desired information. When subjected to an interrogation signal, the IC chip converts said programmed information into a corresponding electromagnetic signal, which is then propagated as radio frequency waves by the antenna.
Typically, a plurality of RFID devices of the type described above are manufactured on a common carrier web, with the antennae of the RFID devices mounted on the carrier web and the IC chips mounted on their respective antennae. The combination of an individual RFID device and its underlying portion of the carrier web is typically referred to in the art as an RFID inlay. The interconnected web of RFID inlays is typically wound by its manufacturer into roll form for shipping and further processing by a customer (the customer often called a “converter”). The converter may, for example, cut individual RFID inlays out of the web and thereafter attach the individual RFID inlays to corresponding objects of interest using plastic fasteners or other attaching means. Alternatively, the roll manufacturer may further process the web of RFID inlays to yield a web of adhesive RFID labels (see, for example, U.S. Ser. No. 10/961,590, which is incorporated herein by reference), which the converter may then dispense as-is onto objects or which the converter may customize by printing text, designs or other markings onto the labels prior to dispensing the labels onto the objects of interest. Typically, an automated dispenser is used by the converter to dispense the labels from the web onto the objects of interest.
As can be appreciated, if one wishes to be able to retrieve the information associated with a desired object, one should not apply a defective RFID device to the object. Unfortunately, the occurrence of defective RFID devices is not trivial, with current estimates of defective RFID devices being as high as about 1–20% of all RFID devices produced. Consequently, it is common for RFID devices to be tested for performance prior to their application to objects. Such testing may be performed by the roll manufacturer prior to shipping the roll to the converter, with defective inlays or defective labels being marked as such by the manufacturer using appropriate printed markings (such as a black dot). Such testing may also be performed by the converter prior to dispensing since defects may occur during shipping or handling of the roll after testing by the manufacturer. It is generally desirable to identify defective inlays as early as possible in the supply chain for reasons of both cost and throughput at subsequent stages.
Currently, the testing of RFID devices is typically performed using either one of two different testing techniques, namely, short-range testing and long-range testing. These two testing techniques reflect the differences in electromagnetic physics that exist in the areas around an antenna. There are three commonly accepted regions around an antenna, namely, (i) the reactive near-field (where antennae operate from zero distance to approximately R<=ë/2(at 915 MHz, ˜52 mm)), (ii) the radiating near-field (where R>ë/2and R<2D^2/ë, where D is the largest dimension of the antenna aperture), and (iii) the radiating far-field (where R>2D^2/ë). Short-range testing involves testing within the reactive near-field, and long-range testing involves testing within the radiating near-field or the radiating far-field. In general, long-range testing is likely to be truer to a real life application, wherein the RFID device is likely to be applied to an object and then interrogated at a range within the radiating near-field or far-field regions. One problem with long-range testing is that, because of the proximity of RFID devices to one another on a common carrier web, the interrogation signal emitted from the tester typically elicits responses from a plurality of neighboring RFID devices, many of these responses then being simultaneously detected by the tester. Because there is currently no way to correlate the responses from the responding RFID devices to the physical locations of the responding RFID devices (since the unique identifiers of the RFID devices typically do not follow a particular sequence and are effectively randomized), even if the reader notes that a defective device is present, there is no way to know which of the responding devices is the detected defective device.
One approach to the foregoing problem has been to position an apertured metal shield over the web so that all of the devices within range of the interrogation signal, except for the one device that is positioned within the aperture, are shielded from the interrogation signal. An example of this approach is disclosed in U.S. Pat. No. 6,104,291, inventors Beauvillier et al., which issued Aug. 15, 2000, and which is incorporated herein by reference. One shortcoming of this approach is that the shielding, itself, represents a departure from the operating conditions to which a device “in the field” is typically exposed and may affect the performance characteristics of the device being tested.
An example of short-range testing is disclosed in International Publication No. WO 2004/072892 A2, which was published Aug. 24, 2004, and which is incorporated herein by reference. In this type of short-range testing, an interrogator is capacitively coupled to the particular device one wishes to test.
Although short-range testing in general overcomes the problem of multiple devices being simultaneously activated by a single interrogation, the behavior of RFID devices in the near-field reactive region is different from that expected in the near-field and far-field radiating regions, and hence, there is less confidence without additional analysis that the test results will be valid to the real life application.
Other documents that may be of interest include the following, all of which are incorporated herein by reference: U.S. Pat. No. 6,784,789, inventors Eroglu et al., which issued Aug. 31, 2004; U.S. Pat. No. 6,721,912, inventors Burger et al., which issued Apr. 13, 2004; U.S. Pat. No. 6,487,681, inventors Tuttle et al., which issued Nov. 26, 2002; U.S. Pat. No. 6,412,086, inventors Friedman et al., which issued Jun. 25, 2002; U.S. Pat. No. 6,275,043, inventors Mühlberger et al., which issued Aug. 14, 2001; U.S. Pat. No. 6,259,353, inventors Berger et al., which issued Jul. 10, 2001; U.S. Pat. No. 6,236,223, inventors Brady et al., which issued May 22, 2001; U.S. Pat. No. 6,058,497, inventor Tuttle, which issued May 2, 2000; and U.S. Pat. No. 5,983,363, inventors Tuttle, which issued Nov. 9, 1999.